The striae of Retzius are incremental growth lines or bands seen in

tooth enamel Tooth enamel is one of the four major Tissue (biology), tissues that make up the tooth in humans and many animals, including some species of fish. It makes up the normally visible part of the tooth, covering the Crown (tooth), crown. The other ...

. They represent the incremental pattern of enamel, the successive apposition of different layers of enamel during crown formation.

There are 3 types of incremental lines: Daily incremental lines (cross striation), striae of Retzius and neonatal lines.

Appearance

When viewed microscopically in cross-section, they appear as concentric rings. In a longitudinal section, they appear as a series of dark bands. The presence of the dark lines is similar to the annual rings on a tree. They are named after Swedish anatomist

Anders Retzius Anders Adolph Retzius (13 October 1796 – 18 April 1860), was a Swedish professor of anatomy and a supervisor at the Karolinska Institute in Stockholm. Biography Retzius was born in Lund to Anders Jahan Retzius and Ulrika Beata Prytz in ...

. In the longitudinal section of a tooth, these lines appear near the

dentin Dentin ( ) (American English) or dentine ( or ) (British English) () is a calcified tissue (biology), tissue of the body and, along with tooth enamel, enamel, cementum, and pulp (tooth), pulp, is one of the four major components of teeth. It i ...

. They bend obliquely near the cervical region. They curve occlusally near the cuspal regions or the incisal regions. Produced during the second stage of enamel calcification, also known as the maturation stage,

ameloblast Ameloblasts are cells present only during tooth development that deposit tooth enamel, which is the hard outermost layer of the tooth forming the surface of the crown. Structure Each ameloblast is a columnar cell approximately 4 micrometers in ...

s produce matrix and enamel at the rate of 4 micrometers per day; however every fourth day there is a change in development. Brownish lines, the striae of Retzius, develop as a result of a change in the growth process. Macroscopically, these lines can be seen on the labial surface or lip side of anterior or front teeth as horizontal lines on the

tooth crown In dentistry, the crown is the visible part of the tooth above the gingival margin and is an essential component of dental anatomy. Covered by Tooth enamel, enamel, the crown plays a crucial role in cutting, tearing, and grinding food. Its shap ...

, also known as

perikyma Perikymata (Greek plural of περικύμα, perikyma) are incremental growth lines that appear on the surface of tooth enamel as a series of linear grooves. In anatomically modern humans, each perikyma takes approximately 6–12 days to form. T ...

ta or "imbrication lines" . Evenly spaced Retzius lines indicate a 6- to 11-day cycle of enamel formation.

Causes

Occasional darker striae or grooves of Retzius result from systemic disturbances in the human body. For example, a

fever Fever or pyrexia in humans is a symptom of an anti-infection defense mechanism that appears with Human body temperature, body temperature exceeding the normal range caused by an increase in the body's temperature Human body temperature#Fever, s ...

can cause some lines to appear darker than those surrounding them. Normally,

amelogenesis Amelogenesis is the process of forming tooth enamel, the hard, protective outer layer of teeth. This process begins during tooth development after the initial formation of dentin ( dentinogenesis), the layer beneath the enamel. The inner enamel e ...

involves a period of enamel matrix formation and a rest period. In case of any disturbance, the rest periods are prolonged and occur close to one another. Consequently, the line of Retzius appears broader and much more prominent, often presenting a brownish colour under the microscope. The

neonatal line The neonatal line is a particular band of incremental growth lines seen in histologic sections of both enamel and dentin of primary teeth. It belongs to a series of a growth lines in tooth enamel known as the Striae of Retzius denoting the prolonged ...

is the darkest band, which represents the disrupted enamel formation due to the stress of being born. It is also said to occur due to periodic bending of

enamel rods An enamel prism, or enamel rod, is the basic unit of tooth enamel. Measuring 3-6 μm in diameter in primates, enamel prism are tightly packed hydroxyapatite crystals structures. The hydroxyapatite crystals are hexagonal in shape, providing rigidit ...

. The formation of the striae of Retzius results from a constriction of

Tomes' process Tomes's processes (also called Tomes processes) are a histologic landmark identified on an ameloblast, cells involved in the production of tooth enamel. During the synthesis of enamel, the ameloblast moves away from the enamel, forming a project ...

es when the activity of

s – cells only present in laying down enamel – is narrowed in conjunction with an increasing process of

interrod enamel Interrod enamel is histologically identified on microscopic views of tooth enamel. Because interrod enamel is located around enamel rods, the areas of interrod enamel enhances the "keyhole" appearance of enamel rods by acting as its border. The loca ...

development. The striae of Retzius often extends from the

Dental-enamel junction The dentinoenamel junction or dentin-enamel junction (DEJ) is the boundary between the enamel and the underlying dentin that form the solid architecture of a tooth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mout ...

to the outer surface, ending in shallow pits known as

perikymata Perikymata (Greek plural of περικύμα, perikyma) are incremental growth lines that appear on the surface of tooth enamel as a series of linear grooves. In anatomically modern humans, each perikyma takes approximately 6–12 days to form. T ...

Qualities

Lines of Retzius (think age bands like tree growth rings) – Stria (A) * artifacts in enamel (not found in

) created by incremental steps of ameloblasts * comparable to the contour "lines of Owen" in dentin * have increased organic content and show the variations in rhythm as the tooth enamel matrix calcifies * follow an appositional or side-by-side growth pattern

Illustrations

Several diagrams and photographs of these lines (and others) appears in a document published by the

Max Planck Institute for Evolutionary Anthropology The Max Planck Institute for Evolutionary Anthropology (, shortened to MPI EVA) is a research institute based in Leipzig, Germany, that was founded in 1997. It is part of the Max Planck Society network. Well-known scientists currently based at ...

on their website:

References

Teeth {{dentistry-stub